In recent years, with widespread use of portable electronic devices such as notebook computers, smartphones, portable game devices, and personal digital assistants (PDAs), the need for reducing the size of secondary cells for use as power sources and increasing the energy density has been growing in order to reduce the weight of these devices and to achieve the use of these devices for a longer period of time.
Particularly in recent years, secondary cells have been more widely used as power sources for vehicles, such as electric vehicles and electric motorcycles. Secondary cells for use also as such power sources for vehicles need not only to have a higher energy density, but also to be capable of operating in a wide temperature range.
Nickel-cadmium cells, nickel-hydrogen cells, and other suitable cells have conventionally been mainstream nonaqueous electrolyte secondary cells. However, to satisfy the demand for reducing the size of secondary cells and increasing the energy density, lithium ion secondary cells tend to be more frequently used.
A lithium ion secondary cell includes electrodes each obtained by coating a current collector with an electrode mixture that contains an active material, a binder, and a conductive assistant, and drying the coating on the current collector.
For example, a positive electrode is obtained by coating an aluminum foil current collector with slurry of a positive electrode mixture in which LiCoO2 serving as an active material, polyvinylidene fluoride (PVdF) serving as a binder, and carbon black serving as a conductive assistant are dispersed in a dispersion medium, and drying the slurry coated on the current collector.
On the other hand, a negative electrode is obtained by coating a copper foil current collector with slurry of a negative electrode mixture in which graphite serving as an active material, carboxymethylcellulose (CMC), styrene-butadiene-rubber (SBR), PVdF, or polyimide serving as a binder, and carbon black serving as a conductive assistant are dispersed in a dispersion medium, and drying the slurry coated on the current collector.